Take a look at the QuickTime movie below. It will show a still image for 10 seconds, then a blank screen. Then it will show you the image again. Your job is to look for a detail that has been changed between the two images.

Most people have difficulty with this task. Even when the part that changes is central to the image, accuracy is typically no better than 50 percent. For the particular type of change depicted in this movie, accuracy averages less than 30 percent. If you didn’t notice the change, drag the slider in the movie quickly back and forth and you should be able to spot the change.

This phenomenon, “change blindness,” has been the focus of a considerable amount of research (we’ve reported on it severaltimesbefore). Researchers have found that changes to “central” objects are easier to spot than “marginal” objects, but there is disagreement about what makes an object central. Is it some physical characteristic of the object itself? Or is it a cognitive process that viewers actively apply to the object?
Pauline Pearson and Evelyn Schaefer have designed an experiment to address this question. They pre-tested a set of 40 photos, asking volunteers to rate changes between the pictures as marginal or central. All the photos were street scenes, so they asked a second set of viewers to rate the changes as relevant or irrelevant to driving. (For example, moving a crosswalk might be relevant, but removing a building could be irrelevant).

For the change blindness task, they displayed a picture for 15 seconds, then a black screen for a half second, then the altered picture for another 15 seconds. This was a different procedure from other change blindness studies we’ve reported on, where the two images flashed repeatedly. Pearson and Schaefer chose this method so they could evaluate whether viewers were better at identifying objects which moved, or objects which disappeared (if pictures flash repeatedly, there is no way to know if observers are noticing the object appearing or disappearing). They found that viewers were less likely to notice central objects relocating than disappearing.

For the next part of the study, they gave different instructions to two sets of viewers. One group was asked only to look for changes between the pictures. The second group was told that they were helping to determine if the set of pictures was effective for evaluating driving ability. Here’s how the first group responded:

As expected, this group was more accurate at identifying central changes compared to marginal changes. They were also better at identifying changes related to driving. Now take a look at the second group’s responses:

For this group, as long as a change was relevant to driving, they were equally accurate at detecting central or marginal changes. Their ability to detect irrelevant changes was similar to the first group, but they were significantly better at noticing marginal, driving-related changes.

For central changes, the meaningfulness of the change does not appear to matter — viewers are no more accurate detecting the change when focused on the driving task. But for marginal changes, the cognitive focus of the viewer has a large effect on their ability to notice change.

I’d add that this result may be a partial explanation for the results of studies on driving with cell phones: as the cognitive demands of a phone conversation increase, driving performance suffers. The brain appears to have a finite capacity for multi-tasking; as the number and complexity of tasks we try to do increases, our performance must necessarily suffer.

I noticed the island change locations too, but it took me several viewings of the video to find it. I never moved the slider, but I guess I created my own sort of slo-mo flicker task by re-viewing the video several times.

Kelley, Chun, & Chua (2003) demonstrated a similar effect: They, like Pearson & Schafer found that central changes were noticed more frequently than marginal changes. However, when the context of the scene was disrupted by turning the picture upside down, participants were much less likely to notice the central changes.

This not only suggests that we notice central changes more, but also that the meaning and context of a scene influences what we deem as “central” or “marginal” — a finding that coincides nicely with Pearson & Schafer’s work.

I didn’t notice the island moving till I went back and forth. I didn’t notice anything the first time I looked; second time I thought the colors of the pools might be more faded. Then moving the slider back and forth, I still thought that might be the case. Is it?

I don’t think those are pools — they are buildings with blue roofs. But you’re right — the color of the roofs does appear to change in the second picture. I didn’t do any manipulation of the colors in photoshop, though, so I think it’s just an artifact of the quicktime video compression. In fact, if you continue to move the slider, you’ll see that the colors gradually change in this “still” image, so that by the end of the video, the roofs are the same bright blue they were at the beginning.

When I watched, I lazily looked over the first image once I realized it wasn’t going to disappear as soon as I thought it would. Then, when the second image popped up, I stared at that until the movie ended, and then I thought that maybe something was up with the position of the island. It kind of looked different, so I put the slider back to a position where the first image was, and it turned out that I was right!

Sorry for double posting, but I thought I should mention that I didn’t notice the roof colors being different. Maybe I wasn’t focusing in the same way on the picture as someone else might. I know I don’t remember seeing the picture as much more than its general features and positions in the landscape. I don’t really know if I even saw the buildings very much as buildings; they kind of looked like rocks at the time of the first picture (in my memory of what was going on in my head, anyway).

When looking at it the first time, I got it right away. When I was staring at the picture, I thought to myself what I thought they would change, and I guessed it would be the island and I was right. Usually this tactic works well if you can guess what you think they’re going to change.